Manufacture of electrical resistance units



July 12, 1927. L. JONES MANUFACTURE OF ELECTRICAL RESISTANCE UNITS FiledOct. 27, 1924 INVENTOR Les+er L. Jones ATTORNEYS the thin metal film.

Patented July 12, 1927.

UNITED STATES LESTER L. J ONES, 01 ORADELI NEW JERSEY.

MANUFACTURE OF ELECTRICAL RESISTANCE UNITS.

Application filed October 27, 1924. Serial No. 746,026.

This invention relates to electrical resistance units and to the methodof making the same; and has special reference to the manufacture ofcapacity and inductance-free high resistances.

As is well known, capacity and inductance-free high resistance elementsare made in the art by any of a number of methods, such as byimpregnating paper strips or other similar material with various kindsof carbon inks, by surfacing paper or the like with graphite streaks orlines, or by depositing thin films of metal on insulating bases such asglass tubes or the like. These resistances, although compact,inexpensive, inductance and capacity-free, have a number ofdisadvantages, the inked and graphited paper resistances being forinstance quite microphonic, generally destructible in use,

- and incapable of carrying any considerable current and the metallictilm resistances being unstable by reason of the change in theresistance value thereof which takes place due to the crystallization oroxidation of These resistance units are also open to the common andserious obresistances of uniform value, especiallywhere the same areproduced by quantity production methods.

The manufacture of resistance units which do not possess theabove-stated objections and disadvantages and which are characterized bybeing non-microphonic, stable in value or magnitude, of large currentcarrying capacity, durable, and capable of being manufactured uniformlyin desired values within wide ranges, is a prime desideratum of mypresent invention.

A further prime object of my present invention comprehends the provisionof a resistance unit of the character referred to provided with a newand improved mounting for connecting the same in circuit, the mountingprotectively encasing the resistance element and being constructed toprovide uniform water-tight electrical and mechanical joints, thestructure being such as to be capable of dissipating the heat generatedwhen the resistance is used for carrying high loads, the saidmountingbeing further characterized by its simplicity of constructionand by the ease with which the parts may be assembled while securingefiicient electrical contact or engagement of the resistance elementwith the circuit connectlng parts.

To the accomplishment of the foregoing and such other objects as mayhereinafter appear, my invention consists in the elements and theirrelation one to the other, as hereinafter particularly described andsought to be defined in the claims; reference being had to theaccompanying drawings which show the preferred embodiment of mylIlVGIltlOIl, and in which: i

F g. 1 is a view with parts shown in crosssection of a resistance unitmade in accordance with the principles of my invention and the mountingtherefor, with parts broken away,

Fig. 2 is a view taken in cross-section on the line 2-2, Fig. 1,

Fig. 3 is a view taken in cross-section on the line 3-3, Fig. 1, and

Figs. 4;8 are views showing in sequence steps of the method of makingthe resistance units or elements.

Before describing my invention in detail, I will briefly premise thatthe resistance unit of my present invention generically comprises a basemade of an insulating material having a surface layer ofgraphite whichis so applied and subjected to such treatment as to produce ahomogeneous and set graphite layer firmly adhering to and embedded inthe body of the base, the resulting unit having the properties ofstability in magnitude or value, of large current-carrying capacity,durability andabsence of any microphonic tendencies. These resistanceunits are made so that after the graphited layer base or blank isproduced, it may be subjected to a further simple operation to produceany t given 'or desired magnitude with accuracy and within a wide rangeof values. The resistance unit of predetermined value is then mounted ina protective casing which is constructed with moisture-proof electricaland mechanical joints and designed so as to be capable of quicklyextracting from within and dissipating the heat generated by theresistance element when the same carries heavy loads.

Referring now more in detail to the drawings, and having reference firstto Figs. 4'? thereof, showing certain steps in the method of making theresistance unit, the resistance element comprises a base or blank I)(Fig. 4) of an insulating material, the said base having a hard andsemi-permeable or porous surface, to which base is applied a layer ofgraphite 9 (Fig. 5) in such manner as to become firmly and intimatelyunited with the body of the base and to form a homogenedus, compactedand stable layerthereover.

- The blank 6 is preferably cylindrical in contour for a purpose whichwill become clearer hereinafter, and consists of unglazed porcelain orclay known in the trade as lavite, the base having a relatively smoothexternal surface free from oil or grease and provided with surface pores30 shown in exaggerated form for purposes of illustration in Fig. 4 ofthe drawings. In produc ing the applied layer of graphite, this blank orbase 6 is first dipped into a solution preferably of colloidal raphiteknown in the trade as Kollag. here resistance units of less than onemegohm are desired, a water soluble solution is used and where theresistance units are to be compact and of values larger than one megohm,an oil soluble solution is preferably used. In either case, the blankafter being dipped in the graphite solution is allowed to soak to permitthe colloidal graphite to be carried by the liquid into the pores of thelavite surface, causing the graphite to become embedded in and firmlyadhered to the body of the base. IVhere a water soluble solution isused, the dipped blank is left to stand for about one half a minutewithout being permitted to dry,

and in the case of the oil soluble solution,

the blank may be left to soak for a longer period of time to allow amore or less deep penetration of the graphite into the pores of theblank.

After the dipping and soaking steps, the adhering graphite layer is thencompacted to effect intimate cohesion between the graphite particles,and this'step is carried out by rapidly burnishing the graphite layer by9. preferably axial action of a smooth metallic surface pressed with afairly highdegree of pressure against the graphite covered blank. Theburnishing motion is preferably rapid and the pressure adjusted to thehighest point short of that which will abrade the lavite. Under theinfluence of this burnishing process, the colloidal graphite iscompacted into a hard shiny mass on the surface and in the top layer ofthe lavite blank. In the use of the water soluble solution, theburnishing step is carried out while the dipped blank is still in amoist state, so as to prevent the compacted graphite from flaking off.

To eii'ect a substantial amalgamation or solidification of the graphitelayer and to- :et the same, the compacted graphite unit is nbjected tothe action of an electric current 1; preferably high voltage. This isaccomished by mounting the blank at its ends an electric circuit ofapproximately 110' volts with a series ballast resistance. Under theinfluence of the electrical current, either from a thermal orelectrolytic action, the graphite becomes densely compacted, formingpractically a-solid layer, and the layer permanently decreases inresistance and becomes set in value. The resistance drop due to thisstep in the process is usually of the order of to and after thisoperation further successive passages of current result merely in thenormal temperature resistance changes due to the ne ative resist- 1 ancecoei'ficient of the graphite, t e resistance coming back to its normalvalue after cooling.

In the resistance unit so obtained, as shown in Fig. 5 of the drawingsand generally designated as u therein, the layer of graphite g ispermanently united with the base, and becomes substantially unitary orintegral therewith, the unit being entirely free from microphonicnoises; and the said unit may be heated .to at least 500 Fahrenheit without alteration. The layer so formed is furthermore substantiallyindestructible, and may be buifed against a cloth wheel withoutappreciably changing the resistance value obtained.

To produce resistances of any given or desired value, the resistanceunit a is then subjected to an operation in which parts of the layer areremoved to produce a resistance path of any given value. This is accom-'plished by mounting the resistance unit in a suitable lathe, asdiagrammatically shown in Fig. 6 of the drawings, operating to produce ahelical groove in the grapite layer for obtaining a helical resistancepath. As shown in Fig. 6, the unit a is received by the end chucks 10and 11, the chucks being preferably provided with fingers which firmlygrip the ends of the unit to make efficient electrical contacttherewith, the chuck 10 being grounded on the base 12 and the chuck 11being insulated therefrom, as by insulatably mounting the tail stock 13on the base, as clearly shown in Fig. 6 of the drawings. For producingthe helical groove in the graphited layer of the unit, a rotary grinder14, whichmay be a fine emery wheel grinder, is used, which rotarygrinder is mounted in suitable bearings supported in a carriage 15,which carriage is moved longitudinally of the machine by means of screwmechanism 16 operated by the meshing pinion and gear 17 and 18, thelatter being fixed to the lathe shaft and operated from any suitablesource of energy by connecting means 19. The rotary grinder 14 may beindependently driven by means of a motor 20 mounted on the mova lecarriage 15. For measuring or indicating the resistance value of theunit operated upon, I provide an ohmmeter 21 in circuit with the unit,one end of the ohmmeter being grounded to the machine base 12 by means iof the conductor 22, and the other end being connected to a source ofenergy 23 which in turn is connected to the tail stock 13.

In operation, it will be understood that the rotary grinder cuts ashallow helical groove in. in the resistance unit until the ohmmetershows the desired resistance value. By changing the pitch of the screwfeed and /or the widths of the cut, the resistance of the unit may bevaried within wide limits. For example, it has been found ractical tochange blanks having an initial terminal resistance of 200 ohms into anhing up to approximately 200,000 ohms. he unit with the helical groovecut therein is shown in Fig. 7 of the drawings. An important advantageof this type of unit resides in the ability to use the same stock withthe same machinery for making units of any predetermined value.

To produce a finished unit, the helical resistance element may beprovided with a protective covering 24, as shown in Fig. 8 of thedrawings, and to the ends of the unit may be secured the terminal caps25 and 26 suitably attached by any means such as a Woods metal filling27.

As heretofore stated, it is a further object of the invention to providea new and improved mounting for the resistance unit constructed toproduce uniform water-tight electrical and mechanical joints. with thestructure such that the heat generated by the resistance unit is quicklydissipated, the mounting being further characterized by its simplicityof construction and the ease with which the parts may be assembled whileobtaining efiicient electrical engagement between the parts. To theseends, and as shown in Figs. 1-3 of the drawings, the resistance unit uis mounted in a tubular casing 28 made of a strong insulating material,such for example as a phenol condensation product. the said casing beingexteriorly threaded as at 29 at its opposite ends (only one threaded endbeing shown in the draw ings), the said threaded ends receiving endterminal caps 30 and 31, these caps being interiorly threaded forreception by the threaded casing, as clearly shown in Fig. 1 of thedrawings. The resistance element u is slightly longer than theinsulating casing 28, the ends thereof projecting into the terminalcaps. as is clearly apparent from Fig. 1 of the drawings. For obtainingefficient electrical connection between the end caps and the ends of theresistance elements, I employ contact members made of relatively softmetal which are wedged into circumferential engagement with the ends ofthe resistance element and the end caps when the latter are threadedlysecured to the casing, such members comprising at each end a lead ring32 received by an end of the resistance unit and wedged into contacttherewith by means of the bevelled surfaces 33 and 34 rovidedrespectively in the end caps and casin ends. The construction is suchthat when the end caps are screwed fast onto the casing, the lead ringsare forced into the triangular space between the bevelled surfaces andinto intimate contact with the end graphoidal surface of the blank andwith the metal end caps. This forms a perfectly uniform and water-tightelectrical and mechanical joint.

To permit the resistance unit to be mounted either in spring clips orwired directly into circuit, the preferred construction includes thepunched metal tabs 35 and 36 secured at each end between the casing andthe metal end caps. These punched tabs, together with the heavy terminalcaps, assist in withdrawing and dissipating the heat generated in theinterior under the high loads which this type of resistance unit maycarry.

The method of making and using resistance units embodying my inventionand the numerous advantages thereof will in the main be fully apparentfrom the above detailed description thereof. It will be further apparentthat numerous changes and modifications may be made in the structuredisclosed without departing from the spirit of the invention. defined inthe following claims.

I claim:

1. The method of making a resistance unit which consists in applying asolution of graphite to a cylindrical base having a hard andsemi-permeable surface, the graphite solution penetrating into the poresof the base and forming a layer thereover, in then operating upon saidlayer to effect an intimate cohesion between the graphite particles forproducing a homogeneous layer, and in then producing a helical groove insaid layer to lengthen the resistance path and to provide a resistanceof a given value.

2. The method of making a resistance unit which consists in applying asolution of graphite to a cylindrical base having a hard andsemi-permeable surface, the graphi'te solution penetrating into thepores of the base and forming a layer thereover, in then compacting andsolidifying the layer, and in then cutting a helical groove in saidlayer to lengthen the resistance path and to produce a resistance of agiven value.

3. A resistance unit comprising a cylindrical base, a layer ofresistance material on said base, the said resistance layer beingrovided with a helical groove for producmg a resistance of a givenvalue.

4. A resistance unit comprising a cylindrical base, a homogeneous layerof graphite intimately united with the surface of said base, the saidlayer being provided with a helical groove for producing a resistance ofa given value.

, ble surface, a homogeneous layer of graphite intimately united withthe surface of the base, the said layer having a helical groove cutthereinto to produce a helical resistance path of predetermined value.

6. A resistance unit comprising a cylindrical base having a hard andsemi-permeable surface, a layer of graphite penetrating the pores of andburnished onto the surface of said base, the said layer being providedwith a helical groove cut thereinto to produce a helical resistance pathof predetermined value.

7. A resistance unit comprising a tubularshaped casing, a cylindricalresistance element therein, terminal end caps secured to the ends of thecasing, and contact rings encircling opposite ends of the resistanceelement wedged between the resistance element and the end caps by thesecuring of the end caps to the casing.

8. A resistance unit comprising a tubular.

casing made of an insulated material, a cylindrical resistance elementwithin the casing, terminal end caps threadedly attached to the ends ofthe casing, and contact rings made of a relatively soft metal wedgedinto circumferential engagement with the ends of the resistance elementand the end caps when the latter are threadedly attached to the casing.

9. A resistance unit comprisinga tubular casing having ends exteriorlythreaded and interiorly bevelled, terminal end caps interiorly threadedand bevelled received by the threaded ends of the casing, a cylindricalresistance element within said casing, and contact rings made of arelatively soft metal at (apposite ends of the resistance element, saicontact rings being wedged between the resistance element andfthebevelled ends of the casing and end caps to make efficient electricalcontact with the end caps and resistance element when the end caps arethreadedly secured to the casing.

10. A resistance unit comprising a tubular casing having ends'exteriorly threaded and interiorly bevelled, terminal end capsinteriorly threaded and bevelled received by caps to make efiicientelectrical contact with the end caps and resistance element when the endcaps .are threadedly secured to the casing.

11. A resistance unlt comprising an elongated base made of an insulatlnmaterial,

a layer of resistance material app ied to said base, the said resistancelayer being provided with a helical groove out along the base forproducing a resistance of a given value.

12. A resistance unit comprising an elongated base of an insulatingmaterial, a thin coating of resistance material applied to said base,the said resistance coating being font with a. helical groove forproducing a helical resistance layer of a given value.

Signed at New York city, in the county of New York and State of NewYork, this 21st day of October, A. D. 1924.

LESTER L. JONES.

